Instructions for heating vapor-tight packages in a microwave oven usually call for first piercing each package with a sharp utensil. See, for example, FIG. 22 of U.S. Pat. No. 4,425,368 (Watkins). Vapor-tight frozen food packages which comprise polymeric or plastic film can be hard to pierce, and one may think that the film has been pierced when it has only been indented. If the film is not pierced, vapor pressures built up during heating may cause the package to explode. Instead of exploding, the package may rip at a seam through which the contents may spill out into the oven.
A number of self-venting, vapor-tight microwave oven packages have been proposed. Each of the packages shown in U.S. Pat. No. 4,013,798 (Goltsos) consists of a compartmented plastic tray across which is sealed a plastic film. A side wall of one or more of the compartments has a notch at which the plastic film is less well sealed so that a buildup of vapor pressure in a compartment breaks the seal at the notch to vent the compartment.
U.S. Pat. No. 4,292,332 (McHam) concerns a vapor-tight package for popping popcorn in a microwave oven. Its top wall is provided with lines of weakness that will begin to rupture at a vapor pressure less than that which would cause the bag to explode.
U.S. Pat. No. 4,141,487 (Faust et al.) concerns a vapor-tight package comprising a plastic film which is formed with a slit along a crease line. The edges of the slit are sealed together by an adhesive sealant material that melts below the cooking temperature to open the slit and thereby release vapors.
U.S. Pat. No. 4,404,241 (Mueller et al.) concerns a vapor-tight package comprising a heat-resistant sheet formed with apertures, and bonded to that sheet is a continuous heat-softening material which extends across the apertures. Rising temperatures and pressures within the package cause the heat-softening material to flow to create vents through the apertures.
U.S. Pat. No. 4,390,554 (Levinson) concerns a vapor-tight, multi-layer microwave oven package including a liquid-barrier plastic film 4 such as nylon or polyester which is "designed to vent at a preselected temperature by blow out plugs 13 or can be constructed of a low temperature plastic (as polyethylene) formulated to melt at a predetermined temperature". See col. 4, lines 30-40, and FIG. 1.
U.S. Pat. No. 4,210,674 (Mitchell) illustrates a tray which is hermetically sealed by a plastic film to which a narrow strip of aluminum foil is adhesively secured. When the aluminum foil has certain dimensions, it converts microwave energy to heat sufficient to melt the plastic film, thus venting the package. When we constructed such a package, the venting did occur, but there was visible and audible arcing which would probably be objectionable to prospective users. Also, it was difficult to adhere such a narrow strip of aluminum foil to a plastic film. Furthermore, many food processors routinely monitor their products to locate any hazardous metal objects, and such an aluminum strip might interfere.
The Mitchell patent suggests at column 3, lines 18-30 that substitutes for the aluminum foil include "silver micropaint", "a copper-filled coating" and "dispersions of metal powder", and that such substituents may be applied by "a printing wheel or a spray applicator".